An independent report suggests the operators of the Latrobe Valley's coal mines have grossly underestimated the cost of rehabilitating the sites once they shut down.

The mine operators have told the Hazelwood Mine Fire Inquiry that Loy Yang's estimated rehabilitation cost is $112 million, Yallourn's is up to $91 million and Hazelwood's is about $73 million.

However, a report prepared for the State Government by technical services company AECOM conservatively estimates the liability cost of closing the mines earlier than planned is $196 million for Loy Yang, $170 million for Yallourn and $251 million for Hazelwood.

The inquiry also heard from an economist, Michael Cramer, who said the $15 million rehabilitation bonds set for the mine operators did not meet the Victorian Government's own guidelines.

The bonds are paid by mining companies as financial assurance that a site can be rehabilitated, even if they fold.